Assay measurement techniques seek to measure the amount of a target analyte in solution by capturing it with, in part, at least one capture agent or capture probe (herein referred to as capture agent). In some cases, a first or primary capture agent is immobilized on a surface, for example, a sensor surface or a micro particle surface. A second capture agent can be used to simultaneously bind (either by specific, non-specific or cross-reactive association) with the analyte, as in a sandwich assay. When no analyte is present in a sample, small amounts of binding of materials in the sample still can result from non-specific surface association. Typically, the amount of bound target analyte is quantified by comparing the magnitude of a signal recorded after both binding and washing steps (e.g., washing away of non-target material) have been performed with a baseline signal recorded prior to exposing the surface to the sample containing the analyte.
The quantitative methods used in conventional assay measurement techniques are limited in accuracy because they do not account for, for example, variation in the output of different apparatus, provide an effective means for providing multiple measurements over the time period that the assay is conducted or provide a reliable means for removing interfering background materials from the apparatus during operation. Further, some assay measurement techniques require elaborate techniques that employ additional hardware to account for variation in the output of different apparatus.
Conventional assay measurement techniques also lack adequate means for identifying assay failures due to, for example, sample preparation errors, improper mixing of fluids, changes in physical properties of assay materials, and variation in fluid flow.
A need therefore exists for improved assay measurement apparatus and methods and quality assurance and quality control methods and apparatus.